Bulk Silica Research Articles

Correlating Double‐Cone Polariton Dynamics with Local Femtosecond Laser Modifications in Ultrashort‐Pulse‐Stimulated Crystals

AbstractStrong similarities for relativistic electrons stimulated from electron beams and electron clouds are merging the boundary of wave‐particle duality of electrons, one of which being superposition is Cherenkov radiations. Recent theoretical investigations into quantum Cherenkov effects for cone‐shape particles have hypothesized electron‐spin flip transitions in bound‐ and free‐electron systems like bulk silica and graphene. However, experimental validation of these predictions remains outstanding. Here, analogous phenomena of polariton‐version quantum Cherenkov radiations observed by broadband femtosecond pump‐probe experiments is reported, and demonstrate double‐cone emission processes involving of Cherenkov‐type phonon polaritons (PhPs) and plasmon‐PhPs in ultrashort‐pulse‐stimulated ferroelectric crystals and graphene, respectively. Through component analysis of double‐cone polariton emissions using an empirical quantum dynamics model, the initial polariton dynamics are restored by time translation corrections. The resulting quantum wave packets of PhPs are found to be correlated with femtosecond‐laser‐induced modifications inside ferroelectric lithium niobates, achieving a PhP threshold criterion for ultrasmooth femtosecond laser nanofabrication.

Precision Macroporous Monoliths Made Using High‐Throughput Microfluidic Emulsification

Materials with pore sizes ranging from micrometers to millimeters find use in thermal insulation, acoustics, separation, and energy‐related applications. While several routes have been developed to manufacture such macroporous materials, current fabrication technologies remain limited in either scalability or pore size control. Herein, a scalable microfluidic approach is reported to create macroporous materials with precisely controlled pore sizes from monodisperse emulsion templates. Emulsion droplets produced in a parallelized step emulsification device are assembled by gravity and converted into macroporous monoliths by polymerization and drying. Microstructural analysis and model filtration experiments show that the pore windows of the bulk macroporous material can be tightly controlled and used for the size‐selective separation of particles from suspensions. By heat treating macroporous structures of selected compositions, one can also create bulk carbon and silica glass monoliths with unique cellular architectures for potential applications as filters, separation membranes, or catalyst supports.

Effect of Liquid Load Level and Binder Type on the Tabletability of Mesoporous Silica Based Liquisolids

Mesoporous silica offers an easy way to transform liquids into solids, due to their high loading capacity for liquid or dissolved active ingredients and the resulting enhanced dissolution properties. However, the compression of both unloaded and loaded mesoporous silica bulk material into tablets is challenging, due to poor/non-existing binding capacity. This becomes critical when high drug loads are to be achieved and the fraction of additional excipients in the final tablet formulation needs to be kept at a minimum. Our study aimed to investigate the mechanism of compression and tabletability dependent on the Liquid Load Level of the silica and type of filler/binder in binary tabletting mixtures. To this end, Vivapur® 101, FlowLac® 90, Pearlitol® 200 SD and tricalcium citrate tetrahydrate were selected and mixed with Syloid® XDP 3050 at various Liquid Load Levels. Compaction characteristics were analysed using the StylOne® Classic 105 ML compaction simulator. Additionally, the Overall Liquid Load (OLL) was defined as a new critical quality attribute for liquisolid tablets. The Overall Liquid Load allows straightforward, formulation-relevant comparisons between various fillers/binders, liquid components, and silica types. Results indicate strong binding capacity and high plasticity of the fillers/binders as key components for successful high liquid load silica tablet formulation. A volumetric combination of 30% Vivapur® 101 and 70% 0.75 mL/g loaded Syloid® XDP 3050 proved to be the most effective mixture, achieving an Overall Liquid Load of 36–41% [v/v] and maintaining a tensile strength of 1.5 N/mm2 with various liquid vehicles.Graphical

4.8-μm CO-filled hollow-core silica fiber light source

Mid-infrared (MIR) fiber lasers are important for a wide range of applications in sensing, spectroscopy, imaging, defense, and security. Some progress has been made in the research of MIR fiber lasers based on soft glass fibers, however, the emission range of rare-earth ions and the robustness of the host materials are still a major challenge for MIR fiber lasers. The large number of gases provide a variety of optical transitions in the MIR band. When combined with recent advances in low-loss hollow-core fiber (HCF), there is a great opportunity for gas-filled fiber lasers to further extend the radiation to the MIR region. Here, a 4.8-μm CO-filled silica-based HCF laser is reported for the first time. This is enabled by an in-house manufactured broadband low-loss HCF with a measured loss of 1.81 dB/m at 4.8 μm. A maximum MIR output power of 46 mW and a tuning range of 180 nm (from 4644 to 4824 nm) are obtained by using an advanced 2.33-μm narrow-linewidth fiber laser. This demonstration represents the longest-wavelength silica-based fiber laser to date, while the absorption loss of bulk silica at 4824 nm is up to 13, 000 dB/m. Further wavelength expansion could be achieved by changing the pump absorption line and optimizing the laser structure.

Dual role of two-dimensional graphene in silica aerogel composite: Thermal resistance and heat node

Incorporating two-dimensional graphene sheets, known for their high in-plane thermal conductivity, into silica aerogel enhances both the thermal insulation and mechanical properties of the composite. This work delves into the impact of graphene doping on the solid thermal conductivity of silica composites and their mechanical characteristics, employing molecular dynamics simulations that encompass both amorphous silica bulk and porous silica aerogel structures. Our findings reveal that graphene–silica interface thermal resistance notably restricts heat conduction in graphene doped silica aerogel, with the most pronounced effect occurring when graphene aligns vertically with the heat flux. Within porous silica aerogel, graphene serves as a “heat node”, bridging and connecting adjacent pores. A notable “trade-off” emerges: high weight percentages of graphene sheets augment heat transfer through the “heat node effect”, whereas the interfacial thermal resistance between graphene and aerogel reduces solid heat transfer, leading to decreased thermal conductivity. Additionally, while the inclusion of graphene sheets offers limited mechanical property enhancements, their two-dimensional structure can cause detachment and slipping from the silica matrix, leading to localized improvements in mechanical strength. This work sets the stage for advancing graphene-based aerogel composites characterized by low thermal conductivity and enhanced mechanical properties.

Gas-Phase Production of Hydroxylated Silicon Oxide Cluster Cations: Structure, Infrared Spectroscopy, and Astronomical Relevance.

The interaction of free cationic silicon oxide clusters, Si x O y + (x = 2-5, y ≥ x), with dilute water vapor, was investigated in a flow tube reactor. Product mass distributions indicate cluster size-dependent dissociative water adsorption. To probe the structure and vibrational spectra of the resulting Si x O y H2 + (x = 2-4) clusters, we employed infrared multiple photon dissociation spectroscopy and density functional theory calculations. The planar rhombic cluster core of the disilicon oxides (x = 2) appears to be retained upon dissociative adsorption of one H2O unit, whereas a significant structural transformation of the tri- and tetra-silicon oxides (x = 3 and 4) is induced, resulting in an increased coordination of the Si atoms and more 3D cluster structures. In an astronomical context, we discuss the potential relevance of Si x O y H z + clusters as seeds for dust nucleation and catalysts for carbon-based chemistry in diffuse or translucent interstellar clouds, where all the necessary conditions for producing these species are found. In the produced clusters, the frequency of the isolated silanol Si-OH stretching vibrational mode is considerably blue-shifted compared to that in hydroxylated bulk silica and small inorganic compounds. This mode has a characteristic frequency range between 1200 cm-1 (8.3 μm) and 1090 cm-1 (9.2 μm) and is associated with the anomalously small Si-OH bond lengths in these ionised species. In infrared observations such high frequency Si-O stretching modes are usually associated with a pure bulk silica component of silicate cosmic dust. The presence of Si x O y H2 + clusters in low silica astrophysical environments could thus potentially be detected via their signature Si-O band using the James Webb space telescope.

Size effects on the fracture behavior of amorphous silica from molecular dynamics simulations

In this work, the role of structure size and interaction potential on the ductility and mechanical properties of bulk glasses are extensively analyzed using molecular dynamics (MD) simulations. Elastic moduli and mechanical properties for bulk silica structures were calculated from the MD trajectories using three different force fields - diffusive charge reactive potential (DCRP), Teter and Vashishta potentials. These results from MD simulations were compared to experimental measurements and the overall results reassert that, while the elastic moduli show a neglectable variation with structure size, the fracture behavior is considerably affected. Specifically, it is found that the length along the deformation axis is the driver for the brittle to ductile transition. The fracture results, combined with an energy analysis, reveal that the energetic condition for brittle fracture, where elastic strain energy should overcome the fracture surface energy, remains valid for the three potentials.

Structure and thermal conductivity of high-pressure-treated silica glass. A molecular dynamics study

High-pressure treatment of oxide glasses can lead to significant alteration of various material properties such as increased density, ductility, and elastic moduli. In this study, a model of melt-quenched bulk silica glass was subject to high-pressure treatments (up to 16 GPa) using molecular dynamics simulations. The thermal conductivity of such prepared glass structures was determined using the equilibrium Green–Kubo method. We observed that, up to the pressure treatments of ∼6 GPa, the structure exhibits moderate density increase and a much steeper increase between 6 and 16 GPa, with associated density increase of fivefold silicon atoms. We also observed a noticeable increase (up to 20%) of the thermal conductivity in samples subjected to high-pressure treatments. The observed increases are somewhat, but not significantly, larger than those predicted by the minimum thermal conductivity model, accounting for density and elastic moduli increase.

Ultrathin Two-dimensional Layered Composite Carbosilicates from in situ Unzipped Carbon Nanotubes and Exfoliated Bulk Silica.

A key task in today's inorganic synthetic chemistry is to develop effective reactions, routes, and associated techniques aiming to create new functional materials with specifically desired multilevel structures and properties. Herein, we report an ultrathin two-dimensional layered composite of graphene ribbon and silicate via a simple and scalable one-pot reaction, which leads to the creation of a novel carbon-metal-silicate hybrid family: carbosilicate. The graphene ribbon is in situ formed by unzipping carbon nanotubes, while the ultrathin silicate is in situ obtained from bulk silica or commercial glass; transition metals (Fe or Ni) oxidized by water act as bridging agent, covalently bonding the two structures. The unprecedented structure combines the superior properties of the silicate and the nanocarbon, which triggers some specific novel properties. All processes during synthesis are complementary to each other. The associated synergistic chemistry could stimulate the discovery of a large class of more interesting, functionalized structures and materials.

Ultrathin Two‐dimensional Layered Composite Carbosilicates from in situ Unzipped Carbon Nanotubes and Exfoliated Bulk Silica

AbstractA key task in today's inorganic synthetic chemistry is to develop effective reactions, routes, and associated techniques aiming to create new functional materials with specifically desired multilevel structures and properties. Herein, we report an ultrathin two‐dimensional layered composite of graphene ribbon and silicate via a simple and scalable one‐pot reaction, which leads to the creation of a novel carbon‐metal‐silicate hybrid family: carbosilicate. The graphene ribbon is in situ formed by unzipping carbon nanotubes, while the ultrathin silicate is in situ obtained from bulk silica or commercial glass; transition metals (Fe or Ni) oxidized by water act as bridging agent, covalently bonding the two structures. The unprecedented structure combines the superior properties of the silicate and the nanocarbon, which triggers some specific novel properties. All processes during synthesis are complementary to each other. The associated synergistic chemistry could stimulate the discovery of a large class of more interesting, functionalized structures and materials.

Single-model uncertainty quantification in neural network potentials does not consistently outperform model ensembles

Neural networks (NNs) often assign high confidence to their predictions, even for points far out of distribution, making uncertainty quantification (UQ) a challenge. When they are employed to model interatomic potentials in materials systems, this problem leads to unphysical structures that disrupt simulations, or to biased statistics and dynamics that do not reflect the true physics. Differentiable UQ techniques can find new informative data and drive active learning loops for robust potentials. However, a variety of UQ techniques, including newly developed ones, exist for atomistic simulations and there are no clear guidelines for which are most effective or suitable for a given case. In this work, we examine multiple UQ schemes for improving the robustness of NN interatomic potentials (NNIPs) through active learning. In particular, we compare incumbent ensemble-based methods against strategies that use single, deterministic NNs: mean-variance estimation (MVE), deep evidential regression, and Gaussian mixture models (GMM). We explore three datasets ranging from in-domain interpolative learning to more extrapolative out-of-domain generalization challenges: rMD17, ammonia inversion, and bulk silica glass. Performance is measured across multiple metrics relating model error to uncertainty. Our experiments show that none of the methods consistently outperformed each other across the various metrics. Ensembling remained better at generalization and for NNIP robustness; MVE only proved effective for in-domain interpolation, while GMM was better out-of-domain; and evidential regression, despite its promise, was not the preferable alternative in any of the cases. More broadly, cost-effective, single deterministic models cannot yet consistently match or outperform ensembling for uncertainty quantification in NNIPs.

Three-dimensional opal-like photonic crystals made of diamond shells by chemical vapor deposition

3D ordered hollow diamond spheres packed into opal structure have been synthesized by chemical vapor deposition using silicon inverse opal template which, in turn, was produced by molding of a bulk silica opal template. These 3D diamond opals demonstrate optical properties of photonic crystal with Bragg reflection peak in the visible range. Numerical modeling of the reflection spectra is performed using a scattering matrix calculation, and agrees well with the measured Bragg peak positions. The porous structure consisting of nanocrystalline diamond shells with thickness of the order of 10 nm composes the light-weight material with density of ≈0.50 g/cm3. The diamond opals showed bright photoluminescence at 738 nm wavelength due to silicon-vacancy centers formation in course of the growth process. The diamond opals can be a new promising platform for versatile applications, primarily in photonics, due to wide transparency window, high chemical resistance and mechanical strength inherent to diamond.

A scattering spectrometer for white light interferometry

White light interferometry is a non-contact method to measure surface topography. The depth profile of the surface is typically encoded in the spectrum of the reflected light, and a spectrometer is used to decode this signal. In this work, we demonstrate a new form of spectral interferometry, a subset of white light interferometry, in which the conventional spectrometer is replaced with an engineered scattering chip and a camera. The chip is created using laser direct writing to controllably embed scattering centres within a bulk silica substrate, forming a highly stable optical scattering element. Calibration of our device is straight-forward: we circumvent the need for spectral characterisation and computational extraction of the spectrum, and instead directly measure the relationship between surface depth and the white light speckle pattern. Using our technique, we demonstrate surface profile measurements with a resolution of 27 nm over a range of 0.5 mm. Our work provides a new route to the development of potentially compact and low-cost white light interferometers with implications for the measurement of distance, strain, temperature, pressure, and more.

Thermal infrared spectral characteristics of martian dust deposits and evidence for atmosphere-regolith interactions

It has been hypothesized that the dust component of the Martian surface is globally homogeneous, based on chemical similarity between landing sites and spectral similarity from select areas within bright regions. We tested that hypothesis by producing the first near-global data set of surface spectral emissivity (excluding polar regions) across the ∼233–508 and 825–1650 cm−1 (∼20–50 and 6–12 μm) spectral ranges from Mars Global Surveyor Thermal Emission Spectrometer data and using various data reduction techniques to search for any spectral heterogeneity in bright regions that might be present. We found no unequivocal evidence for spectral heterogeneity, supporting the hypothesis that dust is globally homogenized. The global emissivity product permits new spectral parameter maps and preliminary assessments of atmosphere-regolith interactions. We produced the first map of the Christiansen feature (CF) and show that, unlike on the Moon, where CF is a proxy for bulk silica content, CF position on Mars is primarily associated with dust cover. We produced an updated map of the 1630 cm−1 emissivity peak that arises from bound H2O in fine-particulate material and show that the peak is nearly ubiquitous across the Martian surface, including in dark regions with relatively low dust cover. This is attributed to minor amounts of dust disproportionally contributing to the spectral signal in the ∼1630 cm−1 region. We show that regions within the equatorial dust deposits with higher annual modeled frequency of nighttime CO2 frosts are more likely to have lower emissivity in the ∼1350-1400 cm−1 region, consistent with a higher fraction of unconsolidated dust. This provides the first spectral evidence for a previously hypothesized regolith gardening process via a diurnal CO2 cycle, representing an important surface-atmosphere interaction that may contribute to near-surface porosity and affect diffusive exchange of H2O between atmosphere and hydrated solids (ice, minerals) in the regolith.

Defect generation mechanisms in silica under intense electronic excitation by ion beams below 100 K: Interplay between radiative emissions

Ion-beam effects on bulk silica at low temperature have been studied with the aim of understanding the routes and mechanisms leading from the initial generation of free carriers and self-trapped excitons (STEs) to the production of two stable defect structures in irradiated silica, non-bridging oxygen hole centers (NBOHCs) and oxygen deficient centers (ODCs). Ion beam induced luminescence (ionoluminescence, IL) spectra were obtained using 3 MeV H, 3.5 MeV He, 19 MeV Si, and 19 MeV Cl ions and a range of cryogenic irradiation temperatures from 30 to 100 K. The kinetic behavior of three emission bands centered at 1.9 eV (assigned to NBOHCs), 2.1 eV (assigned to the intrinsic decay of STEs), and 2.7 eV (assigned to ODCs) reveal the physical origin of these emissions under intense electronic excitation. The creation of NBOHCs is governed by a purely electronic mechanism. The kinetics curve of the NBOHC band shows two main contributions: an instantaneous (beam-on) contribution, followed by a slower fluence- and temperature-dependent process correlated with the concentration of STEs. The beam-on contribution is proportional to deposited ionization energy. The growth of the ODC band is linear in fluence up to around 2 × 1012 cm−2. The growth rate is independent of temperature but proportional to the number of radiation-induced oxygen vacancies per ion, showing, unambiguously, that the 2.7 eV emission can be associated with ODCs created in an excited state.

The location of the chemical bond. Application of long covalent bond theory to the structure of silica.

Oxygen is the most abundant terrestrial element and is found in a variety of materials, but still wanting is a universal theory for the stability and structural organization it confers. Herein, a computational molecular orbital analysis elucidates the structure, stability, and cooperative bonding of α-quartz silica (SiO2). Despite geminal oxygen-oxygen distances of 2.61-2.64Å, silica model complexes exhibit anomalously large O-O bond orders (Mulliken, Wiberg, Mayer) that increase with increasing cluster size-as the silicon-oxygen bond orders decrease. The average O-O bond order in bulk silica computes to 0.47 while that for Si-O computes to 0.64. Thereby, for each silicate tetrahedron, the six O-O bonds employ 52% (5.61 electrons) of the valence electrons, while the four Si-O bonds employ 48% (5.12 electrons), rendering the O-O bond the most abundant bond in the Earth's crust. The isodesmic deconstruction of silica clusters reveals cooperative O-O bonding with an O-O bond dissociation energy of 4.4kcal/mol. These unorthodox, long covalent bonds are rationalized by an excess of O 2p-O 2p bonding versus anti-bonding interactions within the valence molecular orbitals of the SiO4 unit (48 vs. 24) and the Si6O6 ring (90 vs. 18). Within quartz silica, oxygen 2p orbitals contort and organize to avoid molecular orbital nodes, inducing the chirality of silica and resulting in Möbius aromatic Si6O6 rings, the most prevalent form of aromaticity on Earth. This long covalent bond theory (LCBT) relocates one-third of Earth's valence electrons and indicates that non-canonical O-O bonds play a subtle, but crucial role in the structure and stability of Earth's most abundant material.

Chirality logic gates.

The ever-growing demand for faster and more efficient data transfer and processing has brought optical computation strategies to the forefront of research in next-generation computing. Here, we report a universal computing approach with the chirality degree of freedom. By exploiting the crystal symmetry-enabled well-known chiral selection rules, we demonstrate the viability of the concept in bulk silica crystals and atomically thin semiconductors and create ultrafast (<100-fs) all-optical chirality logic gates (XNOR, NOR, AND, XOR, OR, and NAND) and a half adder. We also validate the unique advantages of chirality gates by realizing multiple gates with simultaneous operation in a single device and electrical control. Our first demonstrations of logic gates using chiral selection rules suggest that optical chirality could provide a powerful degree of freedom for future optical computing.

Control of silicon dioxide etching rate in hydrogen microwave plasma by addition of oxygen

Hydrogen plasma treatment of thin films and bulk silica is important process for surface cleaning, smoothing and patterning. We studied etching of SiO2 plates in H2 + O2 microwave plasma at moderate pressures at high temperatures in the range of 790–1300 °C. A low-coherent optical interferometer was used for simultaneous in situ measurement of etch rate (ER) and the SiO2 temperature. We show that even a small addition of O2 (<1%) in microwave H2 plasma can effectively reduce the SiO2 ER due to redox reaction. Using an optical profilometry and atomic force microscopy we observed not only a strong etching inhibition by O2 addition in gas, but a significantly smoother etched surface as well. The activation energies Ea = 128 ± 8 kJ/mol and 276 ± 23 kJ/mol are found for the etching in pure H2 and H2 + 0.4 %O2 plasmas, respectively. An exponential dependence of ER on O2 content in gas is established, particularly, the O2 concentration as small as 2.6 % is sufficient to reduce the ER by 100 times. The effect of oxygen induced etching slow-down is supported by thermodynamic consideration of the surface reactions involved. The experiments were complimented with the plasma diagnostics by optical emission plasma spectroscopy to detect volatile etching products, and other species such as OH, and correlate the Si emission intensity with the measured ER. Our results confirm the efficiency of oxygen to control in a wide range the etch rate and surface topography upon atomic hydrogen etching in a microwave plasma.

The effect of enclosed water–ice pockets on porous silica cluster collisions

Collisions between dust particles play an important role in the initial stages of the formation of rocky planets. In space, silica dust exhibits a porous structure due to its formation history. Outside the ice line, water–ice will be present on and in silica dust particles, such that collisions between water-ice-filled porous silica grains are relevant. Molecular dynamics simulations are used to simulate central collisions of such grains (diameter 44 nm), focusing on the analysis of bouncing events. While naked bulk silica particles bounce at high velocities, empty porous silica particles always stick since plastic deformation of the porous structure adds to energy dissipation during the collision. Ice-filled porous silica particles exhibit bouncing collisions already at smaller velocities than naked bulk silica particles of the same size. Collision-induced heating of the ice in the collision zone and also in the pores assists the bouncing process. The coefficient of restitution shows good agreement with available macroscopic models, if both the bouncing velocity and the energy dissipation during the collision are used as fitting parameters. Our results give insights into the agglomeration and bouncing of heterogeneous silica–ice particles, relevant for energetic collision processes outside of the ice line in protoplanetary and planetary systems.

Atomistic characterization of the SiO2 high-density liquid/low-density liquid interface.

The equilibrium silica liquid-liquid interface between the high-density liquid (HDL) phase and the low-density liquid (LDL) phase is examined using molecular-dynamics simulation. The structure, thermodynamics, and dynamics within the interfacial region are characterized in detail and compared with previous studies on the liquid-liquid phase transition (LLPT) in bulk silica, as well as traditional crystal-melt interfaces. We find that the silica HDL-LDL interface exhibits a spatial fragile-to-strong transition across the interface. Calculations of dynamics properties reveal three types of dynamical heterogeneity hybridizing within the silica HDL-LDL interface. We also observe that as the interface is traversed from HDL to LDL, the Si/O coordination number ratio jumps to an unexpectedly large value, defining a thin region of the interface where HDL and LDL exhibit significant mixing. In addition, the LLPT phase coexistence is interpreted in the framework of the traditional thermodynamics of alloys and phase equilibria.